The present invention relates to the field of variable vacuum capacitors and in particular, but not exclusively, to motorized variable vacuum capacitors.
Vacuum capacitors typically consist of or comprise a vacuum-tight enclosure and a capacitance generating arrangement of conductive surfaces (electrodes) inside the vacuum-tight enclosure. The inner volume is pumped down to a very low pressure (typically lower than 10-6 mbar) and kept low over the entire lifetime of the device (typically years) by the vacuum-tight enclosure. The vacuum ensures rood electrical insulation between the electrodes and very low dielectric losses of the device.
The vacuum-tight enclosure is typically made of two conductive collars (which also serve as the electrical terminals of the device), attached in a vacuum-tight manner to an insulating piece (often a cylindrical-shaped ceramic piece). A vacuum capacitor can be fixed (ie no adjustment of the capacitance value is possible after manufacturing), or it can be made into a variable vacuum capacitor in which the capacitance value can be varied, which is typically achieved by moving one electrode with respect to the other by means of an expansion joint (a bellows, for example). The expansion joint is typically driven by a drive system which also includes a motor and some form of control mechanism. The motor is often constructed as a separate addition to the variable vacuum capacitor. However, a variable vacuum capacitor cannot function without such a means of driving and controlling the variable electrode (and hence the capacitance value).
Most common applications of variable vacuum capacitors include broadcasting (in an oscillation circuit of a high power transmission), as well as plasma controlling processes in the semiconductor, solar and flat panel manufacturing equipment (in so-called impedance matching networks). The adjustment of the capacitance value of a variable vacuum capacitor allows modifying and matching a power supply's output impedance to the application's impedance value.
Any part of an electrical circuit responds to the amplitude and phase of an alternative (AC) current. That response (i.e. how it changes the amplitude and/or phase of the current) is described by the impedance which is (in mathematical terms) a complex number made out of a real part and an imaginary part.
High frequency power supplies are manufactured to have standardized impedance values. The standard impedance is 50 Ohms.
High frequency applications (such as plasma processes) called the “loads” of the circuit, can have any impedance value (a+bj) where a and b can be any real numbers and j is defined as the mathematical number whose square equals −1. Typical semiconductor, solar, or flat panel manufacturing require a succession of various plasma processes, which translates into varying load impedances that must be continuously and dynamically matched to the fixed impedance of the power supply.
In impedance matching networks, the function of the variable vacuum capacitor is therefore to equate the following relations at all times (for all loads generated by the applications):Zpower supply=Zmatching network(C, . . . )+Zload,for any (time-varying) load50+0j=Zmatching network(C, . . . )+a+jb,for any a,b values of a time-varying load
Where Z designates the complex impedance values of the high-frequency circuit part (the part is indicated as an index).
The impedance of the matching network Zmatching network(C, . . . ) is a function of the capacitance value C of the variable vacuum capacitor, and can also be a function of other components of the matching network, such as inductive, or resistive, or other capacitive components.
If the load is not properly matched at all tunes, the electrical power from the supply is not well transmitted into the load. Unwanted consequences include energy dissipation or energy reflected back into the power supply which can lead to its destruction. By appropriately adjusting the value of the variable vacuum capacitor, the impedance of the matching network can be tuned for optimum power transfer from the power supply to the load.
The means of moving the movable electrode (sometimes also called the “variable electrode”) can be a separate addition to the device or can be integrated into the device. When integrated, the variable vacuum capacitor is sometimes explicitly referred to as a “motorized variable vacuum capacitor” In any case when comparing the size or speed or other characteristic of the variable vacuum capacitor device, one should always consider the entire system made of “motor ±variable capacitor device”, as both are required in applications.
Known variable vacuum capacitors typically have a bellows which must serve three functions: it must provide a reliable vacuum seal, it must be capable of extending and contracting to allow movement of the movable electrodes, and it must also carry the electrical high-frequency current from the terminal to the movable electrode. This limits the choice of material for the bellows to very few options, as it must be optimized simultaneously for electrical characteristics and for mechanical characteristics. Even with a good choice of material, the long path of the electrical current along the bellows (high-frequency currents are forced to flow along the surface of conductors, a phenomenon known as “skin effect”) can result in considerable electrical losses inside a very critical part of the device, therefore generating undesired heat and an additional parasitic electrical resistance to the capacitive device. Such elevated temperatures and thermal cycling will reduce the total number of duty cycles of the expansion joint, thereby reducing the operating lifetime of the variable vacuum capacitor.
Japanese patent application JP10284347A proposed a variable vacuum capacitor which makes use of two bellows to mitigate the aforementioned inconvenience. Patent document U.S. Pat. No. 6,473,289 (B1), on the other hand, proposed to eliminate the bellows completely and substitute its fictions with other parts and a different layout of electrodes inside the vacuum enclosure.
Patent application US2005052820A, from the present applicant, proposed the use of two series-connected sets of electrodes arranged adjacent to each other in the radial direction. This arrangement results in a rather large diameter device, because a large space in needed in the plane perpendicular to the movement of the variable electrode (to achieve a reasonably high capacitance value). Such a design is discussed in more detail below, with reference to FIG. 2.
A variable vacuum capacitor described in patent application U.S. Pat. No. 3,611,075 A suffers from the same disadvantage, namely that the two fixed electrodes and the two variable electrodes are positioned next to one another in the radial direction. For a given diameter of the device, the capacitance that can be achieved is thus inferior in those prior designs which do not use series-connected electrode sets. Another inconvenience of these devices disclosed in US2005052820A and U.S. Pat. No. 3,611,075 is that, because the electrode radii of the inner set of electrodes are substantially different from the radii of the outer set of electrodes, it is difficult to manufacture the outer and inner electrode sets to have equal capacitance. One must for example adapt the number of turns and/or the length of the inner electrodes as compared to those of the outer electrode.
A further inconvenience of many prior art variable vacuum capacitors is that the motor must be well insulated from the movable electrodes, because the motor is mounted on or near a high voltage terminal of the device. To avoid high voltage discharges from that terminal on to the motor, and to avoid other electrical, interference between the high voltage terminal and the much lower voltage of the motor it is necessary to use a long insulating part, which adds significantly to the overall size of the device.
The variable vacuum capacitor of an aspect of the invention aims to address these and other problems with prior art devices. It is desirable to provide a variable vacuum capacitor having:
an increased serviceable lifetime,
improved voltage and current handling characteristics as compared to those obtained with prior art devices having similar size and capacitance, and/or
a smaller diameter and/or length (eg having capacitative electrodes which can fit into a smaller cylindrical volume with small cylindrical diameter).
In particular, an aspect of the invention foresees a variable vacuum capacitor comprising:
a vacuum enclosure,
a first variable electrode assembly comprising one or more first static electrodes and one or more first mobile electrodes,
a second variable electrode assembly comprising one or more second static electrodes and one or more second mobile electrodes,
a first electrical connection terminal for providing an electrical connection to the one or more first static capacitor electrodes,
a second electrical connection terminal for providing an electrical connection to the one or more second static capacitor electrodes,
displacement means for displacing the first and/or second mobile electrodes relative to the first and/or second static electrodes respectively, along an axis of the vacuum capacitor,
the variable vacuum capacitor being characterized in that
the first and second electrode assemblies are ganged along the axis such that the first mobile electrode assembly is offset along the axis by a gang offset distance from the second electrode assembly, and
the variable vacuum capacitor comprises mobile electrode linkage means for providing a kinematic linkage between the one or more first mobile electrodes at a first position along the axis and the one or more second mobile electrodes at a second position along the axis, such that a first displacement of the one or more first mobile electrodes along the axis results in a second displacement of the one more second mobile electrodes along the axis.
By arranging the first and second electrode assemblies in to linearly ganged configuration, the diameter of the device can be significantly reduced. It is also possible to avoid the need for any electrical connection to any moving parts such as the mobile electrodes, which means that the bellows are not required to act as electrical conductors and can be made of a material which is more suited to the mechanical function. This in turn can significantly extend the working life of the device.
According to a variant of the variable vacuum capacitor of an aspect of the invention, the mobile electrode linkage means is arranged such that the magnitude of the second displacement is the same as the magnitude of the first displacement. The mobile electrode linkage means can for example be a simple, rigid structure which provides a direct mechanical link between the two sets of mobile electrodes, thus enabling a simple and robust construction and reducing the possibility of stray capacitance due to the linkage geometry.
According to another variant of the variable vacuum capacitor of an aspect of the invention, the mobile electrode linkage means comprises electrical connection means for electrically connecting the one or more first mobile electrodes to the one or more second mobile electrodes. Combining the two functions of mechanically and electrically connecting the mobile electrodes further reduces the complexity of the device.
According to another variant of the variable vacuum capacitor of an aspect of the invention, the displacement means comprises a motor outside the vacuum enclosure, and drive transmission means for transmitting a drive force of the motor through a wall of the vacuum enclosure to the on or more first mobile electrodes inside the vacuum enclosure. Since the bellows and the outer surface of the device is insulated from the electrodes, the motor can be mounted much closer to the device (eg on the outer surface of the wall of the vacuum enclosure), which can significantly reduce the overall size of the device.
According to another variant of the variable vacuum capacitor of the an aspect of invention, motor protection insulation can be included to electrically insulate the motor against a high voltage on the one or more first mobile electrodes.
According to another variant of the variable vacuum capacitor of the an aspect of invention, the motor protection insulation is arranged between the drive transmission means and the one or more first mobile electrodes.
According to another variant of the variable vacuum capacitor of the an aspect of invention, the one or more first mobile electrodes and the one or more first static electrodes are substantially cylindrical and coaxial with the axis, such that the one or more first mobile, electrodes are at least partially interleaved with the one or more first static electrodes, and/or
the second mobile electrodes and the one or more second static electrodes are substantially cylindrical and coaxial with the axis, such that the one or more second mobile electrodes are at least partially interleaved with the one or more second static electrodes.
According to another variant of the variable vacuum capacitor of an aspect of the invention, the one or more first mobile electrodes and the one or more first static electrodes are configured as spiral electrodes, and/or wherein the one or more second mobile electrodes and the one or more second static electrodes are configured as spiral electrodes.
According to another variant of the variable vacuum capacitor of an aspect of the invention, the mobile electrode linkage means comprises a substantially cylindrical element arranged around the outside of the first electrode assembly and arranged coaxially with the one or more first mobile and one or more first static electrodes.
The substantially cylindrical element may be at least partially constructed from an electrode material and arranged sufficiently close to an outer one of the one or more first static electrodes to function at least partially as one of the one or more first mobile electrodes. This refinement offers a simple, robust structure which also contributes to an increase in the maximum variable capacitance of the device.
According to another variant of the variable vacuum capacitor of an aspect of the invention, the substantially cylindrical element comprises open regions, and wherein one or more static electrode support elements extend from the first static electrodes, through the openings, to the wall of the vacuum enclosure.
According to another variant of the variable vacuum capacitor of an aspect of the invention, its insulating parts are made at least partially of a ceramic material.
According to another variant of the variable vacuum capacitor of an aspect of the invention, extensible vacuum sealing means (eg bellows) extend between the first electrode assembly and the wall of the vacuum enclosure, the extensible vacuum sealing means being constructed with such a shape and of such materials that it behaves as an electrical insulator, at least when the variable vacuum capacitor is operating at a high voltage and/or at a high frequency.
According to another variant of the variable vacuum capacitor of an aspect of the invention, the one or more first static and the one or more first mobile electrodes have substantially the same dimensions and spatial configuration as the one or more second static and the one or more second mobile electrodes respectively. This variant has two principal benefits: firstly, that the manufacture of the device can be significantly simplified by only requiring tooling for one electrode configuration, and, secondly, that using identical or similar first and second electrode assemblies results in an even distribution of capacitance between the two assemblies, thereby minimising the voltage on the mobile electrodes, which means that the device can operate at a higher applied voltage.
The figures are provided for illustrative purposes only, and should not be construed as limiting the scope of the claimed patent protection. Where the same references have been used in different drawings, they are intended to refer to similar or corresponding features. However, the use of different references does not necessarily indicate that the features to which they refer are different.